1. Field of the Invention
The present invention relates to waveguides that propagate light at multiple, yet discrete speeds—equivalently, multiple discrete transverse modes—and that transport telecommunications signals, generate or amplify light, transport electromagnetic power, or are used for decorative or display purposes.
2. Description of Related Art
For optical fiber lasers to scale to higher energies and powers, the fibers themselves must have greater cross-sectional areas in order to withstand those energies and powers. However, merely scaling the dimensions of optical fibers leads to systematic problems that can compromise the performance and utility of the very systems that they are intended to service. As an example, fibers of large cross sections can support a large number of spatial modes that scale with the square of the fiber diameter relative to the wavelength. Fibers with many higher-order modes result in “hot spots” in the guided optical beam, which can result in catastrophic optical damage and undesirable nonlinear optical effects. In addition, fibers that support a large number of modes can limit the bandwidth (e.g., the data rate) of communication information. This follows since each mode propagates at a slightly different speed through the fiber, with the result that a short optical pulse will temporally spread over a given length of fiber.
Several approaches to increase the effective cross-sectional area of a fiber have been discussed in the literature. In one case, the fiber is designed to maintain but a single spatial mode as the physical size of the fiber increases, thereby increasing the effective cross-sectional area of the fiber. Although functional in many applications, these large-area single-mode fibers are susceptible to bending losses. In another approach, a fiber can be designed to support many higher-order modes (HOM). These HOM fibers, can, in fact, result in a larger effective cross-sectional area, but, circularly symmetric versions suffer from hot spots that reduce the thresholds for nonlinear artifacts and damage.
The present invention circumvents these limitations. Using so-called field-flattening designs, a well tailored, single-spatial-mode HOM fiber can be realized, without the deleterious effects of hot spots. Moreover, by enabling asymmetric structures with azimuthal modal patterns, the resultant fibers can be designed to realize large effective cross-sectional areas, with a spatially uniform mode, as well as to realize polarization-maintaining fibers. Moreover, by fabricating the fiber with a helical (twisted) preform, the resultant HOM, field-flattened fiber can provide an output field with a specific optical angular momentum state, which has myriad potential applications. In general, the fiber structures described herein can be fabricated to possess optical gain regions, with potential application to long-haul communication links, and high-power beam delivery systems for commercial and defense needs. An added feature is that these amplifying fibers can be designed using uniform gain regions, so that the need for non-uniform doping regions can be obviated.
As noted above, a major problem currently exists when scaling fibers (and waveguides) to support higher-order spatial modes, namely, that these structures can result in hot spots, which can lead to optical damage as well as undesirable nonlinear optical artifacts. In addition, existing scaling design rules can result in fibers with multiple spatial optical modes, each of which possess differing propagation speeds (i.e., modal dispersion). In conventional fibers, this state of affairs can lead to undesirable mode mixing, which can also limit the data rate and/or bandwidth in long-haul optical communication links.
Specifically, in the latter case, as one scales up the mode area in conventional fibers, the modal separation decreases in inverse proportion. This decrease in mode separation is undesirable in that it exacerbates modal mixing, which, as noted above, adversely affects the performance of fibers for beam delivery and communication purposes, among others. Moreover, as the fiber scales in size, the presence of additional modes makes it more and more difficult to launch a beam into a single, desired guided mode without exciting undesirable neighboring modes.
Conventional waveguides have other shortcomings as well. While it is known that the high order modes of circularly symmetric waveguides have larger modal separations than the low order modes of these guides, the high order modes suffer from on-axis “hot spots” which may limit the power or energy those modes can carry, and this detriment may outweigh their modal separation benefit. Specifically, these spatial intensity peaks can result in deleterious nonlinear propagation artifacts, which can induce power or energy to couple from a single desired HOM into other modes, degrading the quality of the beam emitted by the waveguide. That is, the diffraction-limited spot size increases, with the result that the minimal focal size of the beam increases. In addition, as the number of spatial modes increases, undesirable optical hot spots appear in the output beam. Conventional waveguides tend to have a packing efficiency of roughly 50%. It is desirable that waveguides have packing efficiency that approaches 100%. This latter benefit is enjoyed by the now well-known waveguides whose fundamental modes are power-flattened. However, those latter waveguides cannot practically achieve the large modal areas necessary for next generation applications, which, using the teachings herein, are now realizable, using the design rules described herein to realize fibers with field-flattened, single HOM configurations.